The present invention relates to gas turbine engines and, more particularly, to gas turbine engines of the type employing a plurality of combustors for burning fuel with air to produce hot, energetic gasses for impingement upon the blades or buckets of a turbine.
A large gas turbine engine conventionally includes a plurality of combustors within each of which a fuel is reacted with a supply of compressed air to produce a plentiful supply of hot gasses. The hot gasses flow at high speed from the combustors to impinge upon the blades or buckets of a rotatable turbine wheel. The turbine wheel rotates an output shaft and also drives a compressor for producing the supply of compressed air. In some gas turbine engines, for example, aircraft jet engines, the output shaft is omitted. The output of the gas turbine is obtained as an exhaust flow which directly propels the aircraft on which it is located.
The combustors are conventionally disposed in a circle about a perimeter of the gas turbine engine. The combustion reaction zones of all adjacent combustors are joined by crossfire, or crossover, tubes which are essentially open tubular structures through which gas and flame are capable of flowing under the influence of a pressure difference in the combustors to which they are connected.
During startup, the shaft of the gas turbine engine is cranked to starting speed by an external energy source. Then, fuel and air are introduced to all of the combustors. A spark plug in one or two of the combustors is fired to start the combustion reaction. As the combustion reaction begins in a combustor, the pressure therein rises due to the production of hot gas. If a neighboring combustor is unlit, the pressure differential produced by the higher pressure in the lit combustor forces hot gas and flame to flow into the unlit combustor. In this way, each adjacent combustor is lit beginning with the lighting of only one or two combustors.
In theory, once all combustors are lit, their pressures equalize and the flow of gas and flame through the crossfire tubes should stop. In practical gas turbine engines, however, differences in geometry, air flow and fuel metering between adjacent combustors may promote continuous gas and flame flow through the crossfire tube joining them. A small amount of flow through the crossfire tubes aids in balancing the pressures and flows from the combustors. The crossfire tubes are connected to the hottest areas of the combustors wherein temperature of, for example, more than 3000 degrees F. may exist. Although the flow of gas and flame through the crossfire tubes does not affect the operation of the gas turbine engine, if a large pressure difference develops between combustors the high gas and flame temperatures flowing through the crossfire tubes are capable of their rapid destruction.
One method for discouraging continuous gas flow in crossfire tubes employs vent holes through the crossfire tubes. Pressurized air in the plenum surrounding the combustors and crossfire tubes flows inward through the vent holes and both cools any gas flowing in the crossfire tubes and tends to equalize the pressure differential along the length thereof. The reduced pressure differential may preclude crossfire gas flow below a given pressure differential. In addition, the air flowing through the vent holes tends to cool the crossfire tube walls to reduce the temperature thereof.
Although they equalize the pressure differential between the ends of the crossfire tube, the vent holes interfere with the primary function of the crossfire tubes in flame propagation. It is thus desirable either to eliminate the vent holes in the crossfire tubes or to limit the amount of vent air capable of flowing therethrough.
Each end of a crossfire tube is affixed in the wall of its combustor using an inward-directed flange. Such flanges tend to become hot and a method for cooling them is desirable.